This invention relates to doctoring apparatus wherein flexible elongated doctor blades are advanced longitudinally across the surfaces being doctored.
U.S. Pat. No. 4,691,406, the disclosure of which is herein incorporated by reference in its entirety, discloses a doctoring apparatus of the above-mentioned type. The doctor blade has a length greater than the width of the surface being doctored. A blade holder applies an intermediate portion of the blade to the surface being doctored. The blade is movable longitudinally through the blade holder, and has continuing portions which extend in opposite directions beyond the ends of the holder to hydraulically driven pay off and take up reels. Hydraulically actuated clamps act on the continuing blade portions and are adjustable between closed settings preventing relative movement between them and the blade, and open settings permitting such relative movement. A hydraulic drive reciprocates the blade holder. The clamps are opened and closed in timed sequence with reciprocation of the blade holder to achieve longitudinal shifting of the blade in a selected direction across the doctored surface, from one to the other of the reels. This type of "pull through" blade transfer system maximizes efficiency by eliminating lost production time normally associated with the changing of conventional "cut to length" blades.
It is extremely vital to the optimum performance of the above-described pull through blade transfer system that the blade be kept under proper tension both on the pay off and take up runs between the reels and the clamps. Too little tension can result in uncontrolled expansion of the coils being removed from or accumulated on the reels, whereas excessive tension can have a damaging effect on the transfer equipment and/or the blade itself, in extreme cases causing blade breakage.